Conventional image capturing apparatuses that capture images in all directions or spherically employ a hyperboloidal mirror or a fisheye lens, for example. These image capturing apparatuses are specific optical systems that need a non-linear image processing such as distortion correction or projective transformation so that the captured image with the apparatus can be visualized to a user.
One of known technologies is that a semispherical image is obtained with a wide-angle capture employing a fisheye lens and recorded as a distorted circular image. A part of the distorted circular image is cut out and image processing is performed on the image with a computer, whereby the distorted image is transformed into a plane regular image.
Another known technology is that while performing the image processing above, when the center position of the distorted circular image does not correctly correspond to the vertical point direction, a user specifies a parameter of a tilt angle. The load while performing distortion correction on the image is therefore reduced.
As described above, when images are captured in a state of the image capturing apparatus being tilted, an omni-directional spherical image in the wrong vertical direction may be generated. Some inventions to solve this problem have been already known.
Japanese Patent Application Laid-open No. 2003-223633 and Japanese Patent Application Laid-open No. 2006-059202 disclose a technology of generating an omni-directional spherical image having the correct vertical direction. This is achieved by adding rotational transformation according to the tilt of a camera to the process of non-linear image processing.
Japanese Patent Application Laid-open No. H11-309137 and Japanese Patent No. 4175832 disclose a technology of generating an omni-directional spherical image having the correct vertical direction at high speed, for the purpose of reducing the time required for generating the omni-directional spherical image having the correct vertical direction or reducing the cost on arithmetic operations. This technology is achieved by adding rotational transformation according to the tilt of the camera, in addition to distortion correction or projective transformation, preparing a conversion table used for non-linear transformation in advance, and performing batch transformation through the conversion table when capturing images.
With the structure of previous spherical image capturing apparatuses, however, users are required to specify a tilt angle of the apparatus during capture when combining a plurality of images captured at once from a plurality of image capturing directions, when transforming the combined image into a plane regular image, and when displaying the resulting image. That is to say, there was a problem in that a necessary parameter for correction could not be obtained during capture for automatically correcting the image.
Additionally, with the previously performed method for capturing images in all directions or spherically through a conversion table according to the tilt, if the amount or orientation of the tilt changes, the conversion table needs to be re-made from the beginning. In this case, processing requires a relatively long time to generate an omni-directional spherical image having the correct vertical direction according to any tilt of the image capturing apparatus.
Japanese Patent Application Laid-open No. H11-309137 and Japanese Patent No. 4175832 disclose in particular a technology generating images using a conversion table that includes rotational transformation according to the tilt of the camera. The conversion table is made so as to correspond to the amount of tilt determined in advance. Accordingly, if the amount or orientation of the tilt changes so as to differ from the predetermined amount or orientation, the conversion table needs to be re-made from the beginning. Also in this case, the processing still requires a relatively long time to generate an omni-directional spherical image having the correct vertical direction according to any tilt of the image capturing apparatus, therefore the problem is not yet solved.
Therefore, there is a need to provide an image capturing apparatus capable of generating an omni-directional spherical image having the correct vertical direction, according to any tilt of the image capturing apparatus.